Tone generator circuits



July 3, 1962 R. c. BURGENER ETAL 3,042,831

TONE GENERATOR CIRCUITS Filed June 27, 1958 A m M m 7 .J 2 B J a a k. 6 0 Z I r m 0 I\/|L|- 2 2 4 I 2 2 I IMI u j v 4 W. W 3 7 2 "2 d 3 a 3 H 2 W l W 0/0 :SzEzz 2:25;: E551 fi :V \l@ O 8 nlu I 2 2 m m 4 w m, 7 8 3 9 w W M A, NW L wn imi OH 2 i m wa Rm m Y B atent @fiiice 3,042,881 Patented July 3,, 1962 3,042,881 TONE GENERATOR CIRCUITS Robert C. Burgener, Hilliards, and Alexander Finlay, Columbus, Ohio, assignors, by mesne assignments, to C. G. Conn, Ltd., Elkhart, 11111., a corporation of Indiana Filed June 27, 1958, Ser. No. 745,023 Claims. (Cl. 331-117) This invention relates to tone generating circuits for electronic musical instruments such as electric organs and more particularly to an oscillatory tone generating circuit employing a transistor as the amplifying means.

Electronic musical instruments such as organs are conventionally provided with a plurality of oscillators each producing signals having a frequency characteristic of an organ note. Such oscillators have included a tuned inductance :and capacitance circuit connected to vacuum tube amplifying means to maintain oscillation.

In the electronics industry generally, there has been a growing tendency to use transistors in amplifier and oscillator circuits because they are very small and 'have a long operative lifetime. This is particularly important in the construction of electronic musical instruments where it is desirable to have the electronic components occupy as little space as possible. Heretofore, however, it has proved impractical to employ a transistor oscillator in an electronic musical instrument because of the difliculty of stabilizing such a circuit against frequency variations with temperature.

Another problem is that oscillator circuits for generating musical tones are usually actuated and deac-tuated by the manual opening or closing of a key. It is important that the rise and decay times of the signals generated by such circuits be controlled so that undesirable auditory side effects known as key-click or thump are eliminated.

It is an object of this invention to provide a compact and reliable tone generator for use in electronic musical instruments.

It is a further object of the invention to provide a temperature stable oscillator circuit for use as a tone generator in which a transistor is employed as an amplifier.

It is still another object to provide a transistor oscillator circuit useful in generating tones for a musical instrument and in which the circuit is operated by closing a key but in which undesirable auditory effects due to opening and closing of .the key are eliminated.

A feature of the invention is the provision of a degenerative feedback network in the emitter circuit of a transistor used as the amplifying means in the oscillator in order to stabilize the output frequency of the circuit against temperature changes effecting electrical characteristics of the transistor.

Another feature of the invention is the connection of the base and emitter electrodes of a transistor to taps on an inductor coil of a resonant circuit to better match the output impedance of the inductor with the input impedance of the transistor in order to stabilize the frequency of the resonant circuit against temperature change. A conventional coil tapped at various positions determined by the frequency desired may be used in the various tone generating circuits of an instrument.

A further feature of the invention is the provision of a shunt capacitor in the base circuit of a transistor in a tone generating circuit in order to bypass high frequency parasitic oscillations which may be developed due to the tapping of the inductor coil.

Still a further feature of the invention is the combination of manual keying means in the collector circuit of the transistor of a tone generating circuit with a serially connected storage capacitor to provide satisfactory signal rise and decay characteristics and thus eliminate undesirable key-click.

In the accompanying drawings:

FIG. 1 is a schematic circuit diagram of a tone generator circuit in accordance with one embodiment of the invention;

FIG. 2 is a schematic diagram of a similar circuit in accordance with another embodiment of the invention in which the circuit is actuated by the closing of a key; and

FIG. 3 is a schematic diagram of a circuit similar to that of FIG. 2 showing the addition of a-pulse-forming network.

In accordance with the present invention, a transistor is used as an amplifier to sustain oscillations in a resonant circuit tuned to the desired frequency. The base and emitter of the transistor are connected to taps on the inductor coil of the resonant circuit in order to match the input impedance of the transistor to the output impedance of the inductor thus avoiding undesirable shunting through the transistor which would tend to reduce the Q of the resonant circuit. A serially connected resistor in the emitter circuit provides degenerative feedback tending to stabilize the output frequency of the resonant circuit against temperature changes effecting electrical characteristics of thetransistor. A manually operated key is provided to actuate the circuit when it is desired to provide a particular tone. The key may include a switch of either the normally open or the normally closed variety. In using a normally open key switch which closes to actuate the.circuit, it is desirable to provide a storage capacitor in the collector circuit to control the rise and decay time of the signal produced by closing the key and thus eliminate undesirable auditory effects such as keyclick.

FIG. 1 illustrates a tone generator circuit in accordance with one embodiment of the invention which circuit is used to generate a sine wave output and a pulse wave output of a predetermined frequency corresponding to a particular musical tone. In an electric organ, a plurality of such tone generator circuits are employed, one corre sponding to each tone to be produced. The circuit in cludes the capacitor 10 and the inductor coil 11 connected in parallel to form a resonant or tank circuit resonating at a selected frequency. Power for sustaining oscillation of the resonant circuit is provided by the transistor 12 which includes base electrode 13, emitter electrode 14 and collector electrode 16. Because the impedance of the transister 12 is relatively low in comparison to the coil 11, it is desirable to tap the coil at points 17 and 18 to provide base and emitter connections respectively and to match the output impedance of the coil to the input impedance of the transistor. Base 13 is connected to the coil 11 through coupling capacitor 19. Resistor 20 is connected in series between the emitter 14 and the coil tap 18. Bias for the transistor base 13 is provided by the battery 21 and the resistor 22. Shunt resistor 23 is connected across the resonant circuit and is in series with the normally closed key 24 which in turn is connected to ground. The circuit is actuated by opening key 24 and disconnecting the shunt resistor 23.

A sine wave of predetermined frequency is delivered to the output 26 through the isolating resistor 27 connected to the coil 11. A pulse output is delivered to the output connection 28 being developed across the resistor 29.

In operating the generator to produce sine wave and pulse'outputs of predetermined frequency which are in turn used to produce a musical tone, the key 24 is opened to disconnect the resistor 23 so that the current in the resonant circuit including tuned capacitor 10 and induc tor coil 11 begins to oscillate. Since the current fed back to the inductor 11 through resistor 20 induces a voltage across resistor 20 out of phase with the voltaoaassr a; age between taps 17 and 18, it tends to cancel any changes in output frequency of the resonant circuit resulting from changes in the electrical characteristics of transistor 12 due to temperature changes.

Tapping of the inductor coil 11 tends to cause the circuit to break into high frequency parasitic oscilla tions in some instances so that it is desirable to include in the base circuit a bypass capacitor 31 which serves to short out such oscillations to ground.

In a tone generator circuit adapted to resonate at a frequency of 2000 cycles per second, the coil 11 had 4000 turns tapped at 1000 turns for emitter connection 18 and at 2000 turns for base connection 17. In this circuit components of the following values were used:

Transistor 12 General Electric 2N19l. Capacitor 19 0.0068 microfarad. Resistor 20 7000 ohms.

Resistor 22 6.8 megohms.

Resistor 23 68,000 ohms.

Resistor 29 330 ohms.

Battery 21 22 volts.

Capacitor 31 330 microfarads.

In operating an electric organ it is generally preferable to actuate the tone generating circuits by the closing of a switch rather than by opening one as shown in the circuit of FIG. 1. The present invention can be adapted to operate by switch closing by modifying it in accordance with the circuit shown in FIG. 2. In this circuit actuation is accomplished by keying the collector circuit through the key 32. In this embodiment it has been found desirable to connect a storage capacitor 33 and a resistor 34 in parallel with the key 32 and the battery 21. Thus, when the key 32 is closed a discrete time interval is required to charge the capacitor 33 preventing an undesirable sudden rush of current which would cause an undesirable auditory side effect known as keyclick. Similarly the gradual discharge of capacitor 33 when key 32 is opened controls the decay time of the tone so that it is not abrupt and does not cause a thump.

In circuits of the type illustrated in FIG. 2, it has been found that a desirable pulse output is not obtained by simply connecting a resistor to the coil 11 as in the embodiment shown in FIG. 1. For this reason, when a pulse output is desired, a pulse forming network of the type shown in FIG. 3 is incorporated into the circui-t. In this embodiment of the invention, a part of the sine output is taken off at connection 36 and the remainder of the sine output passes through the isolating resistor 37 and the isolating capacitor 38 to the resistor 41. The rectifier 3S3 conducts when the sine wave has a predetermined amplitude to convert the sine wave signal into a pulse wave developed across resistor 4-2. The resistor 42 of the pulse forming circuit serves as a summing resistor and is common to other tone generating circuits in the instrument.

The tone generating circuit of the present invention provides a reliable means for producing oscillations of a frequency corresponding to selected musical tones. A transistor is employed as the amplifying means so that frequent replacement is not required and important space saving advantages are obtained. The provision of the combination of degenerative feedback and impedance matching tapping for the transistor electrode connections gives it good properties of frequency stability against temperature changes. In addition, the circuit may be keyed without production of undesirable auditory effects such as key-click or thump.

We claim:

1. A tone generator circuit for a musical instrument including in combination, a resonant circuit having an inductor, a transistor having emitter, collector and base electrodes, said inductor having a pair of intermediate taps thereon defining a section having an impedance substantially less than the total impedance of said inductor,

means connecting said emitter and said base electrodes to different taps on said inductor, said means including a resistor connected in series between said emitter electrode and said inductor to provide negative feedback in the emitter circuit to compensate for temperature changes in said transistor, transistor biasing means connected to said base and collector electrodes and including a resistor connected in series with said base electrode, a capacitor connected in parallel with the last-named resistor to bypass high frequency parasitic oscillations, keying means coupled to said biasing means for selectively rendering said transistor conductive to sustain oscillations in said resonant circuit, and an output connection to said inductor for deriving an oscillating tone signal therefrom.

2. A tone generator circuit for a musical instrument including in combination, a resonant circuit having an inductor, a transistor having emitter, collector and base electrodes, said inductor having a pair of intermediate taps thereon defining a section having an impedance substantially less than the total impedance of said inductor, means connecting said taps individually to said emitter and said base electrodes, a resistor connected in series between said emitter electrode and said inductor to provide negative feedback in the emitter circuit to compensate for temperature changes in said transistor, transistor biasing means connected to said base electrode, manually operated keying means connecting said collector electrode to a potential source and adapted to close an energizing connection to actuate said circuit, serially connected capacitor means and resistor means connected in parallel with said keying means to control decay time of a signal in said circuit, and an output connection to said inductor for deriving an oscillating tone signal therefrom.

3. A tone generator circuit for a musical instrument including in combination, a resonant circuit having an inductor, a transistor having emitter, collector and base electrodes, said inductor having a pair of taps intermediate the ends thereof defining a section having an impedance substantially less than the total impedance of said inductor, means connecting said emitter and said base electrodes of said transistor to said taps on said inductor, a resistor connected in series between said emitter electrode and said inductor to provide negative feedback in the emitter circuit to compensate for temperature changes in said transistor, transistor biasing means connected to said base and collector electrodes, keying means coupled to said biasing means for selectively rendering said transistor conductive to sustain oscillations in said resonant circuit, an output connection to said inductor for deriving a sinusoidal oscillating tone therefrom, and a pulse forming network including rectifier means and resistor means connected to said inductor for converting a part of the sinusoidal output to a pulse output.

4. A tone generator circuit for a musical instrument including in combination, a resonant circuit including an inductor having a pair of taps intermediate the ends thereof defining a section having an impedance substantially less than the total impedance of said inductor, a ransistor having emitter, collector and base electrodes, means connecting said emitter and said base electrodes of said transistor to separate taps on said inductor, said means including a resistor connected in series between said emitter electrode and said inductor for providing degenerative action, transistor biasing means connected to said base and collector electrodes and applying bias potentials thereto, means connected to said base electrode for bypassing transient oscillations, keying means coupled to said biasing means for selectively rendering said transistor operative to sustain oscillations in said resonant circuit, and an output connection to said inductor for deriving an oscillating tone signal therefrom.

5. A tone generator circuit for a musical instrument including in combination, a resonant circuit having an inductor, a transistor having emitter, collector and base electrodes, said inductor having a pair of intermediate taps thereon defining a section having an impedance substantially less than the total impedance of said inductor, means connecting said emitter and said base electrodes to diiferent taps on said inductor, said means including a first resistor connected in series between said emitter electrode and said inductor to provide negative feedback in the emitter circuit to compensate for temperature changes in said transistor, transistor biasing means connected to said base and collector electrodes and including a second resistor connected in series with said base electrode, a capacitor connected in parallel with said second resistor to by-pass high frequency parasitic oscillations, keying means for selectively rendering said transistor conductive to sustain oscillations in said resonant circuit, a third resistor connected in series with said resonant circuit across which pulse signals are de- 2,764,643 Sulzer Sept. 25, 1956 2,841,711 Koch July l, 1958 2,906,959 Peterson Sept. 29, 1959 2,924,784 Peterson Feb. 9, 1960 OTHER REFERENCES Transistor Radios, by Queen, in Radio Electronics, September 1956, page 91.

Broadcast-Band Test Osc. .lay Fleming in Radio and Television News, October 1955, pages 48-50. 

